1. Field of the Invention
This invention generally relates to a thin film transistor (TFT) and a method of forming the same. More particularly, the present invention relates to a TFT with non-transparent conductive structures under conducting wires that electrically couple with a drain/source, and a method of forming the TFT by using a mask with a slit pattern.
2. Description of the Prior Art
TFTs have been broadly used in contemporary electronic products such as switching on/off pixels in a liquid crystal display (LCD) panel. Hence, the topic of how to improve the structure and forming method for TFT has been popularly discussed.
A well-known TFT structure, as shown in FIG. 1, at least includes following units over a substrate 10: a conductive structure 11, a first dielectric layer 12, a first semi-conductive layer 13, a second semi-conductive layer 14, and a second dielectric layer 15 that is electrically coupled with a conducting wire 16 (patterned conductive structure). Herein, the conductive structure 11 is used as a gate. The first semi-conductive layer 13 is used as a channel and the second semi-conductive layer 14 can be used as a drain or a source. The first dielectric layer 12 and the second dielectric layer 15 are used as insulating and protective material. The conducting wire 16 is used in electrically coupling the drain with external circuits. However, FIG. 1 did not illustrate conducting wires, which electrically couple the gate and the source with external circuits. Basically, the source electrically coupled with external circuit via the conducting wire in the second dielectric layer, and the gate electrically coupled with external circuit via the conducting wire through the first and the second dielectric layer.
Obviously, five masks and pattern-transferred processes are essential for forming the TFT while each patterned layer requires corresponding mask and process, since the patterns of the conductive structure 11, the first semi-conductive layer 13, the second semi-conductive layer 14, the second dielectric layer 15 and the conducting wire 16 are different from each other. Hence, many techniques for reducing required masks and processes have been continually brought out in order to save the material cost, to shorten the process time, and to improve the production ability.
For example, in a prior art, two masks were combined to form the one for forming the second semi-conductive layer 14 and the conducting wire 16 with the same material. However, the optimum cannot be achieved by using the same material since the requirements between the conducting wire and the source/drain are different. In another prior art, two masks were combined to form the one for forming the second dielectric layer 15 and the first semi-conductive layer 13. By doing so, the problems such as higher leakage current (Ioff) and drop height may be resulted from the structure of TFT size. In still another prior art, two masks were combined to form the one for forming the first semi-conductive layer 13 and the second semi-conductive layer 14. However, the practical application can be quite difficult since the special exposure technique is required for the half tone mask. As to these techniques mentioned above can refer to as followings: A. Van Calster et. al. “A Simplified 3-Step Fabrication Scheme for high Mobility AMLCD Panels”; K. Ono et. al. “A Simplified 4 photo-Mask Process for 34-cm Diagonal TFT-LCDs” IDRC 1995; and Chang W. H. et. al. “A TFT Manufactured by 4-Msks Process with new Photolithography” IDRC 1998.
Besides, the light is transmitted to the TFT from the back of the substrate 10 while the TFT is used in a LCD panel, and the first dielectric layer 12 in a prior art is commonly the transparent material, hence the first semi-conductive layer 13, the second semi-conductive layer 14 and the conducting wire 16 may induce the light current (such as the electron-hole pairs excited in semiconductor by the light) resulting to the problem of leakage current or noise.
As mentioned above, there is much space for improving a well-known structure and forming method for TFT. Hence, these problems of how to reduce the required mask in forming TFT process and how to prevent TFT from inducing the light current need to be solved.